He said the "passive millimetre wave scanners" - which QinetiQ helped develop - probably would not have detected key plots affecting passengers in the UK in recent years.

QinetiQ is one of a number of companies that manufacture this kind of security scanning equipment.

'Layered' approach

Mr Wallace told BBC Radio 4's Today programme: "The advantage of the millimetre waves are that they can be used at longer range, they can be quicker and they are harmless to travellers.

"But there is a big but, and the but was in all the testing that we undertook, it was unlikely that it would have picked up the current explosive devices being used by al-Qaeda."

He added: "It probably wouldn't have picked up the very large plot with the liquids in 2006 at Heathrow or indeed the... bombs that were used on the Tube because it wasn't very good and it wasn't that easy to detect liquids and plastics unless they were very solid plastics.

"This is not necessarily the big silver bullet that is somehow being portrayed by Downing Street."

A spokeswoman for QinetiQ said "no single technology can address every eventuality or security risk".

"QinetiQ's passive millimetre wave system, SPO, is a... people-screening system which can identify potential security threats concealed on the human body. It is not a checkpoint security system.

"SPO can effectively shortlist people who may need further investigation, either via other technology such as x-rays, or human intervention such as a pat-down search."

Staff screening

Simon Davies, director of the human rights watchdog Privacy International, also expressed doubts that the scanners would make air travel more secure.

"These machines can't tell you what the object is underneath or within the clothing," he said.

"They can only detect the irregularity. The problem is the way modern clothing is designed, the fact that people take many objects of a non-metallic nature through airports means that the machines are of extremely limited value."

I would prefer to see technology doing the electronic pat down, than a person doing it

Chris Yates Aviation security analyst

He said emphasis was needed to continue with "ordinary and quite boring measures that actually do work" such as screening airport staff and conducting vehicle checks.

Aviation security analyst Chris Yates said better training of staff was needed alongside the introduction of new technology.

"I've seen some very awful examples of the pat down," he said.

"If it's done effectively, yes, you can do a proper examination of somebody and pretty much determine whether they are hiding something.

"But at the end of the shift, on a bad day at work, the security guards just wanting to get home, is he going to want to do that? That's the big issue and I would prefer to see technology doing the electronic pat down, than a person doing it."

Norman Shanks, former head of security for BAA, said body scanners can only be part of the solution and that passenger profiles are also vital.

"Profiling takes into account their behaviour patterns, their tickets, how they purchase them, how they're acting and interacting with people and many times it's believed this is a security person's function."

'Pat down'

The US is also introducing tougher checks for air passengers from nations deemed to have links with terrorism.

BBC transport correspondent Tom Symonds said that, according to the Department for Transport, the new security measures introduced at UK airports on Monday were not causing serious disruption.

People stand fully clothed in a scanner while their image is examined

British Airways said it had no delays and the checks were largely being carried out at departure airports around the world in countries on a list published by the US Transportation Security Administration, our correspondent added.

These include Nigeria, Pakistan, Syria, Iran, Sudan and Yemen.

Mr Brown had said travellers to and from British airports would see the "gradual" introduction of the use of full body scanners and hand luggage checks for traces of explosives.

They will initially operate alongside metal detectors, and be used for all flights in and out of the country.

Scanner technology

On Sunday, Mr Brown accepted there was no way to be certain that the devices would be 100% effective, and "we have got to go further".

The £80,000 full body scanners produce "naked" images of passengers.

They work by beaming electromagnetic waves on to passengers while they stand in a booth. A virtual three-dimensional image is then created from the reflected energy.

The machines are currently being trialled at Manchester airport following tests at Heathrow airport from 2004 to 2008.

They are also being rolled out across the US, with 40 machines used at 19 airports.

The latest decisions came after Nigerian Umar Farouk Abdulmutallab, now in custody, was accused of trying to detonate a bomb on a plane bound for the US on 25 December.

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